This invention relates to laser processing of circuit links and, in particular, to a laser system and method employing a laser beam and substrate positioning system that incorporates a steering mirror to compensate for stage positioning errors and enhance link severing throughput.
Yields in integrated circuit (xe2x80x9cICxe2x80x9d) device fabrication processes often incur defects resulting from alignment variations of subsurface layers or patterns or particulate contaminants. FIGS. 1, 2A, and 2B show repetitive electronic circuits 10 of an IC device or workpiece 12 that are typically fabricated in rows or columns to include multiple iterations of redundant circuit elements 14, such as spare rows 16 and columns 18 of memory cells 20. With reference to FIGS. 1, 2A, and 2B, circuits 10 are also designed to include particular laser severable circuit links 22 between electrical contacts 24 that can be removed to disconnect a defective memory cell 20, for example, and substitute a replacement redundant cell 26 in a memory device such as a DRAM, an SRAM, or an embedded memory. Similar techniques are also used to sever links to program a logic product, gate arrays, or ASICs.
Links 22 are designed with conventional link widths 28 of about 2.5 microns, link lengths 30, and element-to-element pitches (center-to-center spacings) 32 of about 8 microns from adjacent circuit structures or elements 34, such as link structures 36. Although the most prevalent link materials have been polysilicon and like compositions, memory manufacturers have more recently adopted a variety of more conductive metallic link materials that may include, but are not limited to, aluminum, copper, gold nickel, titanium, tungsten, platinum, as well as other metals, metal alloys such as nickel chromide, metal nitrides such as titanium or tantalum nitride, metal suicides such as tungsten silicide, or other metal-like materials.
Circuits 10, circuit elements 14, or cells 20 are tested for defects. The links to be severed for correcting the defects are determined from device test data, and the locations of these links are mapped into a database or program. Laser pulses have been employed for more than 20 years to sever circuit links 22. FIGS. 2A and 2B show a laser spot 38 of spot size diameter 40 impinging a link structure 36 composed of a link 22 positioned above a silicon substrate 42 and between component layers of a passivation layer stack including an overlying passivation layer 44 (shown in FIG. 2A but not in FIG. 2B) and an underlying passivation layer 46 (shown in FIG. 2B but not in FIG. 2A). FIG. 2C is a fragmentary cross-sectional side view of the link structure of FIG. 2B after the link 22 is removed by the laser pulse.
FIG. 3 is a plan view of a beam positioner travel path 50 performed by a traditional link processing positioning system. Because links 22 are typically arranged in rows 16 and columns 18 (representative ones shown in dashed lines), the beam position and hence the laser spots 38 are scanned over link positions along an axis in a first travel direction 52, moved to a different row 16 or column 18, and then scanned over link positions along an axis in a second travel direction 54. Skilled persons will appreciate that scanning may include moving the workpiece 12, moving the laser spot 38, or moving the workpiece 12 and the laser spot 38.
Traditional positioning systems are characterized by X-Y translation tables in which the workpiece 12 is secured to an upper stage that moves along a first axis and is supported by a lower stage that moves along a second axis that is perpendicular to the first axis. Such systems typically move the workpiece relative to a fixed beam position or laser spot 38 and are commonly referred to as stacked stage positioning systems because the lower stage supports the inertial mass of the upper stage which supports workpiece 12. These positioning systems have excellent positioning accuracy because interferometers are typically used along each axis to determine the absolute position of each stage. This level of accuracy is preferred for link processing because the laser spot size 40 is typically only a little bigger than link width 28, so even a small discrepancy between the position of laser spot 38 and link 22 can result in incomplete link severing. In addition, the high density of features on semiconductor wafers results in small positioning errors potentially causing laser damage to nearby structures. Stacked stage positioning systems are, however, relatively slow because the starting, stopping, and change of direction of the inertial mass of the stages increase the time required for the laser tool to process all the designated links 22 on workpiece 12.
In split-axis positioning systems, the upper stage is not supported by, and moves independently from, the lower stage and the workpiece is carried on a first axis or stage while the tool, such as a fixed reflecting mirror and focusing lens, is carried on the second axis or stage. Split-axis positioning systems are becoming advantageous as the overall size and weight of workpieces 12 increase, utilizing longer and hence more massive stages.
More recently, planar positioning systems have been employed in which the workpiece is carried on a single stage that is movable by two or more actuators while the tool remains in a substantially fixed position. These systems translate the workpiece in two dimensions by coordinating the efforts of the actuators. Some planar positioning systems may also be capable of rotating the workpiece.
Semiconductor Link processing (xe2x80x9cSLPxe2x80x9d) systems built by Electro Scientific Industries, Inc. (xe2x80x9cESIxe2x80x9d) of Portland, Oreg. employ on-the-fly (xe2x80x9cOTFxe2x80x9d) link processing to achieve both accuracy and high throughput. During OTF processing, the laser beam is pulsed as a linear stage beam positioner passes designated links 12 under the beam position. The stage typically moves along a single axis at a time and does not stop at each link position. The on-axis position of beam spot 38 in the direction travel 52 does not have to be accurately controlled; rather, its position is accurately sensed to trigger laser spot 38 to hit link 22 accurately.
In contrast and with reference again to FIG. 3, the position of beam spot 38 along cross-axes 56 or 58 is controlled within specified accuracy as the beam positioner passes over each link 22. Due to the inertial mass of the stage or stages, a set-up move to start an OTF run produces ringing in the cross-axis position, and the first link 22 in an OTF run cannot be processed until the cross-axis position has settled properly. The settling delay or setting distance 60 reduces processing throughput. Without a settling delay (or, equivalently, a buffer zone of settling distance 60) inserted before the first laser pulse, several links 22 would be processed with serious cross-axis errors.
Although OTF speed has been improved by accelerating over gaps in the link runs, one limiting factor on the effectiveness of this xe2x80x9cgap profilingxe2x80x9d is still the requirement for the cross axis to settle within its specified accuracy. At the same time, feature sizes, such as link length 30 and link pitch 32, are continuing to decrease, causing the need for dimensional precision to increase. Efforts to further increase the performance of the stage or stages substantially increase the costs of the positioning system.
The traditional way to provide for two-axis deflection of a laser beam employs a high-speed short-movement positioner (xe2x80x9cfast positionerxe2x80x9d) 62, such as a pair of galvanometer driven mirrors 64 and 66 shown in FIG. 4. FIG. 4 is a simplified depiction of a galvanometer-driven X-axis mirror 64 and a galvanometer-driven Y-axis mirror 66 positioned along an optical path 70 between a fixed mirror 72 and focusing optics 78. Each galvanometer-driven mirror deflects the laser beam along a single axis. U.S. Pat. No. 4,532,402 of Overbeck discloses a stacked stage beam positioning system that employs such a fast positioner, and U.S. Pat. Nos. 5,751,585 and 5,847,960 of Cutler et al. disclose split-axis beam positioning systems in which the upper stage(s) carry at least one fast positioner. Systems employing such fast positioners are used for nonlink blowing processes, such as via drilling, because they cannot currently deliver the beam as accurately as xe2x80x9cfixedxe2x80x9d laser head positioners.
The split-axis nature of such positioners may introduce rotational Abbe errors, and the galvanometers may introduce additional positioning errors. In addition, because there must be separation between the two galvanometer-controlled mirrors, the mirrors cannot both be located near the entrance pupil to the focusing optics. This separation results in an offset of the beam that can degrade the quality of the focused spot. Moreover, two-mirror configurations constrain the entrance pupil to be displaced farther from the focusing optics, resulting in an increased complexity and limited numerical aperture of the focusing optics, therefore limiting the smallest achievable spot size. Even assuming such positioners could be used for link-severing, the above-described spot quality degradation would cause poor quality link-severing or incomplete link-severing and result in low open resistance across severed links 22.
What is still needed, therefore, is a system and method for achieving higher link-processing throughput while maintaining focused spot quality.
An object of the invention is, therefore, to provide a system and/or method for achieving higher link-processing throughput while maintaining focused spot quality.
Another object of the invention is to employ a two-axis steering mirror to correct for linear stage settling errors.
Yet another object of the invention is to provide a positioner system employing coordinated motion for semiconductor link processing applications.
This invention preferably employs a two-axis steering mirror, pivotally mounted at the entrance pupil of the focusing lens, to perform small-angle motions that deflect the laser beam enough to compensate for cross-axis settling errors on the order of tens of microns. Although the settling errors occur in both axes, one embodiment of this invention is concerned primarily with correcting settling errors in a cross-axis direction to the OTF direction of linear stage travel. A two-axis steering mirror is employed for these corrections because either axis of the linear stage may be used as the OTF axis. The beam steering mirror is preferably used for error correction only and does not require coordination with or modification of the linear stage position commands, although such coordination is possible.
At least three technologies can be used to tilt a mirror in two axes about a single pivot point. These technologies include fast steering mirrors (xe2x80x9cFSMsxe2x80x9d) that employ a flexure mechanism and voice coil actuators to tilt the mirror, piezoelectric actuators that rely upon deformation of piezoelectric materials to tilt a mirror, and deformable mirrors that employ piezoelectric or electrostrictive actuators to deform the surface of the mirror. Piezoelectric actuators are preferred.
Advantages of the invention include the elimination of cross-axis settling time, resulting in increased throughput particularly for SLP systems. The invention also facilitates improved manufacturability of the main positioning stage(s) due to relaxed servo performance requirements because the steering mirror can correct for linear stage errors.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceed with reference to the accompanying drawings.